In a communications system supporting MIMO (multiple-input multiple-output) technology, to implement downlink beamforming or spatial multiplexing, a base station needs to learn of channel state information of a downlink channel. In a TDD (time division duplex) system, frequency domain resources occupied by an uplink channel and a downlink channel are consistent, and reciprocity exists between the uplink channel and the downlink channel. Therefore, a base station can obtain channel state information of the downlink channel by receiving an uplink reference signal sent by a terminal. For reasons such as transmit power and implementation complexity, a quantity of antennas of the terminal that can simultaneously send a signal is usually less than a quantity of antennas that can simultaneously receive a signal. In this case, the base station can obtain channel state information of only some channels between the base station and the terminal by using the uplink reference signal sent by the terminal. For example, for the terminal, a quantity of antennas that can simultaneously receive a signal is four, and a quantity of antennas that can simultaneously send a signal is one. The base station can obtain channel state information of only ¼ of downlink channels between the base station and the terminal by using the uplink reference signal sent by the terminal.
To obtain channel state information of more channels, the terminal may send the uplink reference signal by using different antennas at different time points separately to obtain channel state information of channels between the base station and the terminal. Based on this method, although channel state information of more downlink channels can be obtained, switching between antennas causes an additional delay, performance loss, and cost increase, and occupies more time-frequency resources used for the uplink reference signal.